The invention relates to a surgical navigation system, comprising a microscope to which a 3D measuring apparatus is rigidly fixed, for localising objects within the field of operation. Markers are fixed to the objects the position of which is desirable to know, for example, the patient or surgical instruments, which are themselves connected to the measuring system. The change in coordinates between the measuring system and the microscope is fixed as the two devices are rigidly connected. Also, knowing the position of the markers relating to the measuring system, it is possible to calculate the change in coordinates between the markers and the image from the microscope. When the markers are recorded, in other words when the relationship between the real objects and the virtual objects is determined, the information can then be used as a navigational instrument and/or for enhanced reality. A typical application for enhanced reality is the superimposition of a 3D anatomical model of the patient on the microscope image, generated from a pre-operative scan of the patient.